"This certainly is in violation of their commitments to the six-party framework," Robert Wood, a state department spokesman, said.
Pyongyang had been disabling the facilities under a disarmament-for-aid deal reached last year with Washington.
The North Korean statement said that the country would "consider restoring the Yongbyon facilities to their original states" but did not say when it would do so.
In June, the cooling tower of the Yongbyon nuclear facility was destroyed in a public demolition to demonstrate North Korea's commitment to scrapping its nuclear programme.
Removal from Washington's list of state sponsors of terror is one of the key concessions offered to North Korea in exchange for shutting down and disabling the reactor.
After North Korea turned over a list of its nuclear programmes in June, the US announced Pyongyang would be delisted.
North Korean 'threats'
The two sides have been negotiating on how to verify the nuclear declaration, with Washington saying it would remove Pyonyang only after a verification plan is agreed.
North Korea's announcement is being seen as a bargaining tactic rather than an intent to restart its nuclear facilities.
Al Jazeera's Melissa Chan said: "In the past, [North Korea] has made a lot of threats on this issue.
"When it comes to negotiating, they really do like to use threats against the other five parties."
China, Japan, South Korea, the US and Russia have all been involved in brokering the deal disablement deal.